Do minor platforms count? I suspect Sailfish has higher native UI adoption. IIRC back when I had a Sailfish phone, all the apps I used including the third-party ones used the same standard UI elements.
They probably meant out of the platforms Apple competes with. Presumably I could find a platform with, say, 12 users and only 1 non-native app, and if that app followed native UI guidelines I could say it has 100% native UI adoption and thus the “highest” out of every platform ever, but that is a useless statement.